The invention relates to a process for preparing dry methacrolein, and to a process for making methyl methacrylate.
Methacrolein (MA) is a common intermediate in methyl methacrylate (MMA) production. MA can be produced from the more abundant ethylene (C2) feedstock, such as via liquid phase propionaldehyde condensation as disclosed in U.S. Pat. No. 4,496,770, or from less abundant isobutylene or tert-butanol (C4) feedstocks, such as via vapor phase C4 oxidation as disclosed in U.S. Pat. No. 5,969,578. The methacrolein product stream contains significant amounts of water for both C2- and C4-based methacrolein production methods. However, water is detrimental to the subsequent oxidative esterification process, such as is disclosed in U.S. Pat. No. 5,969,178, U.S. Pat. No. 6,107,515 and U.S. Pat. No. 6,040,472, which converts methacrolein to methyl methacrylate in a single step, thus advantageously bypassing the intermediate methacrylic acid (MAA) production step of other known processes. Thus, if an MA stream from conventional processes is to be used as a feed stream for a downstream oxidative esterification process, it advantageously is first dehydrated.
U.S. Pat. No. 5,969,578 describes a method for dehydrating a gaseous methacrolein-containing product stream produced by a C4 oxidation process. Water contained in the gaseous stream is removed by partial condensation, while the methanol and methacrolein components are allowed to remain gaseous. The gaseous methacrolein-containing mixture is then contacted with a very dry cooled methanol-containing stream to absorb the methacrolein. The resulting absorber bottoms product mixture contains 25-69% methacrolein in methanol, and <1% water. This mixture is sufficiently dry to feed a downstream oxidative esterification reactor. However, this scheme suffers from a number of disadvantages. For example, it carries large loads of non-condensables through multiple columns, thus requiring unfavorably large equipment, can contain large absorbent recycle streams between multiple upstream and downstream units, requires pure methanol or very dry methanol-containing absorbent streams, produces relatively dilute methacrolein, and can be sensitive to process upsets due to its heavy recycle integration between multiple units. Furthermore, while such a scheme can accommodate a dilute gaseous methacrolein stream resulting from a C4 oxidation process, it is not practical for highly aqueous methacrolein streams produced in a C2-based process, such as the propionaldehyde-formaldehyde condensation process described in U.S. Pat. No. 4,496,770. In fact, U.S. Pat. No. 5,969,578 discourages the use of methods involving the use of water as a contacting solvent for methacrolein production and goes to significant lengths to entirely avoid the co-condensation of water and methacrolein.
Another method of treating a gaseous methacrolein-containing product of a C4 oxidation process is described in U.S. Pat. No. 3,597,880, which discloses a process in which the gaseous methacrolein containing mixture is contacted with methanol to absorb methacrolein and water. The resulting liquid mixture contains methanol, methacrolein, and a modest amount of water, e.g., 5.8% indicated in Example 1. The liquid mixture is then subjected to extractive distillation with water, where the concentration of water in a liquid phase in an absorbing section of the extractive distillation zone is controlled to a concentration of from 50 to 90 mole %. The above concentration of water is targeted in order to avoid the formation of a methacrolein-methanol azeotrope, and to enable the separation of methacrolein from excess methanol that is recycled for use in the upstream absorption step. Methacrolein is thus recovered as a top distillate, whereas the bottom liquid of the extractive distillation column is further distilled to separate methanol from water. The top distillate stream of the extractive distillation column, which is an azeotrope of water and methacrolein (azeotropic point 63.6° C.; methacrolein/water weight ratio 100/7.9, per U.S. Pat. No. 5,969,578) containing about 7% water, is allowed to phase separate to give an organic liquid phase containing mostly methacrolein and a level of water dictated by liquid-liquid equilibrium, about 3% water. The aqueous liquid phase contains mostly water and a level of methacrolein dictated by liquid-liquid equilibrium, namely about 6% methacrolein. The process, however, is not concerned with drying the methacrolein product further, thus producing methacrolein containing approximately 3% water. The patent states that the methacrolein product can be further purified by additional distillation or the like. The process is intended to capture methacrolein from a gaseous C4 oxidation product stream, with the associated absorber bottoms containing mostly methanol, methacrolein, and only a modest amount of water, e.g., 5.8% water indicated in Example 1. This process is not practical for a highly-aqueous, e.g., 65% water, liquid methacrolein product stream of a propionaldehyde condensation process, such as described in U.S. Pat. No. 4,496,770, used with C2 feedstock.
U.S. Pat. No. 2,514,966 and U.S. Pat. No. 2,514,967 disclose a method in which a gas that contains acrolein (or other unsaturated aldehydes) and steam is absorbed into water. This method is carried out by scrubbing an acrolein-containing gaseous mixture with a large amount of water under high pressure to form an aqueous solution containing about 2 wt % of acrolein, and then subjecting the aqueous acrolein solution to stripping, rectification and extractive distillation to recover the acrolein. Such a process is disadvantageous in that an extremely large amount of water must be used under high pressure to absorb acrolein from the gaseous reaction mixture, due to the inherently low solubility of acrolein in water (21.4 wt % under 20° C. per U.S. Pat. No. 3,957,880). This problem would be exacerbated if this method were to be applied to methacrolein, as methacrolein is even less soluble in water (6.1 wt % at 25° C.). Another disadvantage of this method is that in the acrolein separation step, an extractive distillation, is conducted at a temperature below 35° C. using a very large amount of water as a solvent. The large amount of water is required to avoid a heterogeneous extractive distillation zone, and the low temperature is required to achieve a favorable relative volatility regime for acrolein separation that otherwise would not be attained. Such operating conditions require a pressure as low as 50 mmHg. The utility cost of the acrolein separation is high due to the large amount of water used. The capital cost of the acrolein separation process is also high due to the large column diameter required to process the high water flow. These costs would be further exacerbated if this extractive distillation method were to be applied to methacrolein, since even more water (at least 94 wt % of a mixture containing methacrolein) would be required to avoid a heterogeneous extractive distillation zone due to the low solubility of methacrolein in water. Even if implemented, such a scheme would be limited by the MA:water azeotrope to yield azeotrope-like water levels in product methacrolein. Specifically, methacrolein would be recovered in the form of an azeotropic mixture of methacrolein and water [azeotropic point: 63.6° C. & 100:7.9 per U.S. Pat. No. 5,969,578]. Thus, U.S. Pat. No. 2,514,966 and U.S. Pat. No. 2,514,967 teach the importance of avoiding heterogeneous extractive distillation configurations, thus advocating high levels of water.
No desirable process for dehydrating a heavily aqueous methacrolein stream encountered in a C2 based process, such as propionaldehyde condensation, has been described despite the fact that C2 is a more abundant feedstock and would provide an economic advantage in many regions of the world. In view of the deficiencies of the C4-based prior art and the lack of suitable methods to efficiently dehydrate heavily aqueous C2-derived liquid methacrolein streams, it would be desirable to have an improved dehydration process for a C2-derived liquid methacrolein stream.